Second, yes, sadly, you will need to accumulate 900 points to buy the ability to have sigpics from the Store... It didn't used to be like that, but 'tis the way of things now - you might want to take a look at this thread where you can ask (nicely) for donations of credits from other members to speed you on your way to 900 creds

edit/ Also, it might be useful to have a read of this post by one of the mods to give you a better idea of how the credits/points system works, and what each item in the Store is or does